JP2010-114039A discloses a system which includes a bleed passage branched off from an air passage upstream of a fuel cell and joining an air passage downstream of the fuel cell and which bleeds a part of air supplied from an air compressor. In this system, a rotation speed of the air compressor is reduced when the air flowing into the fuel cell is reduced in amount. However, since the rotation speed of the air compressor is not suddenly reduced, excessive drying of the fuel cell is prevented by allowing excess air to escape to the bleed passage.